1. Field of the Invention
This invention relates to the field of electrical switches and particularly to the field of remotely controlled electrical switches capable of interrupting the application of a source voltage to a load in response to an overload condition.
This invention relates more particularly to the field of solid state power controllers for ac (alternating current) and dc (direct current) service or solid state electrical switches such as solid state relays capable of detecting overload conditions and being able to interrupt the electrical service to the load automatically thereby preventing or limiting further damage to the service or load or lead wire and having the additional capability of being remotely resettable after clearing the overload condition. Typical applications for the invention universal solid state power controller include marine and aircraft as well as industrial applications requiring noise free closure and interruption of source power, compact size, extended switch life, low power dissipation and high reliability.
2. Description of the Prior Art
Electromechanical switching devices using solenoid driven means to transfer electromechanical contacts to apply a source voltage to a load, such as relays are well known. When coupled with an electromechanical circuit breaker, a relay provides a remotely controllable power control function capable of supplying ac or dc service to load via its closed contacts in series with an electromechanical circuit breaker. Electromechanical switching devices, such as relays, provide a very low voltage drop at the switch closure thereby affording low power dissipation.
As used in ac power control applications, mechanical contacts cannot conveniently be timed to apply power to the load as the source voltage crosses zero voltage, or to interrupt the application of power as the load current passes through zero so as to minimize transient disturbances. Mechanical wear, electrical arcing and slow response also limit the application of electromechanical devices to applications requiring low closing cycle rates.
The solid state relay is known to overcome the problem of applying a source voltage to a load as the source voltage crosses zero voltage. Solid state relays are also able to interrupt load current as the load current crosses zero. However, solid state relays introduce a substantial voltage drop at the point of circuit closure, thereby providing relatively high power dissipation in comparison to electromechanical switching devices. For this reason, solid state relays find their greatest application in controlling electrical service to loads requiring relatively low load currents.
Circuit breakers used to protect the electrical service for a load are typically complex mechanical devices that rely on magnetic circuits or thermally actuated means to detect an overload condition. Environmental effects such as vibration, moisture, temperature and age along with operational effects such as thermal and mechanical fatigue from past overload cycle experiences contribute to the degradation of these assemblies. Contact arcing that is sustained by a high voltage source as the contacts part to protect a circuit experiencing overload conditions, contribute to circuit breaker contact degradation, and produce transient noise disturbances that can contribute to error logic signal generation.
Solid state power controllers are typically designed for ac or dc applications. Original designs are prepared for each application, and in some cases, for each individual current range. Each design requires a separate investment of time and funds. The design of an solid state power controller for dc service is typically treated as the design of a product entirely different in function from the function of a solid state power controller designed for ac applications.